Two primary objectives outlined in this proposal are concerned with (1) the cellular factors and mechanisms that underlie the programming of plant tumor cells for a form of terminal cellular differentiation that results in the loss of their neoplastic properties, and (2) the chemical structure and mode of action of two representatives of a new class of naturally occurring growth-regulating substances that are importantly involved both with the development of autonomy in plant tumor cells and with inducing a form of terminal cellular differentiation in those cells. These substances have been found to be purinones that contain hypoxanthine as their base and glucose as the sugar moiety. They owe their biological activities to the fact that they are potent inhibitors of cAMP (3':5') phosphodiesterases and, thus, regulators of intracellular levels of cAMP (3';5'). The precise role that cAMP (3';5') plays in the regulation of both cell division and cell differentiation is being studied in model systems that appear to be admirably suited for making such determinations.